EFFECTS OF IRRADIANCE AND SPECTRAL QUALITY ON SEEDLING DEVELOPMENT OF2 SOUTHEAST-ASIAN HOPEA SPECIES

Seedling developmental responses to understory shade combine the effec ts of reductions in irradiance and changes in spectral quality. We stu died the seedling development of two Southeast Asian dipterocarp trees in response to differences in irradiance (photosynthetic photon flux density, PPFD) and spectral quality (red to far-red ratio, R:FR). The two species, Hopea helferei and H. odorata, are taxonomically closely related but differ in their ecological requirements; H. helferei is mo re drought-tolerant and typically grows in more open habitats. Seedlin gs were grown in six different replicated shadehouse treatments varyin g in percentage of solar PPFD and R:FR. The two species differed in th e influence of light variables on most seedling characters, particular ly for final height, internode distance, branch/trunk internodes, stem length/mass, leaf area/stem length, petiole length, and growth/mol of photons received. Most of the characters in both taxa were primarily influenced by PPFD, but spectral quality also influenced some characte rs - more so for H. odorata. The latter species grew more rapidly, par ticularly in the low PPFD treatments, and its leaves were capable of h igher photosynthesis rates. However, growth in H. helferei was not red uced in direct sunlight. The growth of this taxon may be constrained b y adaptations, particularly in leaves, for drought tolerance.